Sealing between static and high speed rotating parts in a gas turbine engine presents a formidable challenge. Rotational speeds in excess of 30,000 rpm for gas generator rotors in small engines is not uncommon. In order that seal wear is minimized, seal elements can be spaced apart allowing for a small controlled amount of leakage flow; however, differing rates of thermal growth between rotor and static structures during transient operating conditions directly affects intercomponent clearances. Additionally, there are rotor eccentricities which must be accounted for as well as a limited amount of mechanical inertial growth. In order to provide an acceptable degree of sealing for both steady state and transient operating conditions, labyrinth seals are commonly used. Typically, the sharp toothed portion of the seal is mounted on the rotating component with an abradable honeycomb or other sacrificial material disposed radially outward to from the land. During certain transient operating conditions clearances are reduced. When there is interference, oftentimes referred to as negative clearance, the sharp teeth wear permanent grooves in the land. As a direct consequence, once steady state operation has resumed, while the clearance between the rotor and stator has returned to nominal, leakage through the seal has increased. Excessive rubbing may cause overheating, degradation and wear of the rotating seal teeth themselves, further increasing leakage. When designing with labyrinth seals, those skilled in the art must compensate for seal degradation by providing sufficient margin in the secondary flow system. In a blade cooling circuit in a new engine, for example, more cooling air than required will be supplied to the blades. Upon degradation of the seal, however, sufficient secondary flow will remain so that hot primary flow gas is not ingested into blade cooling circuits. This additional secondary flow required is not available for combustion and represents a direct parasitic loss in engine efficiency.
Another problem with labyrinth seals is that the complex machining of the seal teeth is typically one of the final operations performed on costly rotating parts which are subject to very high stresses during operation. Any errors which occur at this point could cause the part to be scrapped, or require repair, both at significant cost. The seal teeth are also prone to damage due to mishandling during engine assembly or disassembly.
Another apparatus employed for sealing between high speed rotating and static parts is brush seals. These seals are comprised of a plurality of compliant bristles which extend generally radially inwardly from an annular ring to which they are permanently affixed. The ring is clamped or otherwise fixedly attached to a static member and circumscribes and is concentric with a rotating member or shaft. The brush seal is sized so that the bristles are biased against the shaft, often being canted in the direction of rotation of the shaft. During operation, the bristles rub against the shaft, compliantly deforming due to thermal growth and orbiting of the shaft, thereby maintaining their sealing capability. Unlike permanently grooved labyrinth seal lands, brush seals retain their sealing effectiveness after periods of reduced clearance.
A fundamental problem with conventional brush seal applications however is wear of the bristles. Significant effort has been expended by those skilled in the art in an attempt to define the optimal orientation, size, number, packing density and material of the bristles as well as the amount of dimensional interference with the rotating member so as to provide reasonable sealing effectiveness while achieving an acceptable level of seal wear. Such combinations however fail to address the rudimentary issue that persistent rubbing induces wear. Further, to prevent wear damage to or overheating of the highly stressed rotating member, the land on which the bristles rub is typically coated with a layer of hard ceramic or other protective material so that substantially all wear occurs in the seal bristles. So while the brush seal, due to its compliant nature, initially affords improved sealing over labyrinth seals, bristles wear results in increasing leakage with time. Further, it is contemplated that when worn, the brush seal affords poorer sealing capability than a similarly worn labyrinth seal, the leakage path for the latter being generally longer and more tortuous. As such, the use of conventional brush seal apparatus requires substantially similar excess design margin in the secondary flow as labyrinth seals, so that no permanent gain in overall engine efficiency can be realized.